1. Field of the Invention
This invention relates to welded wire termination and more specifically to a wire termination tab that is integrally formed with a conductive surface, and which is formable into a tubule for receiving a wire for welded termination.
2. Description of the Related Art
In many applications such as satellite solar arrays and printed circuit (PC) boards wires must be terminated to a conductive pad to interconnect circuitry and other electrical systems. Typically, the wire is soldered flat to the conductive pad. However, in high reliability thermally stressful environments solder is not a viable option. Because of the mismatch between the thermal coefficients of expansion (TCE) for the solder, the wire, the conductive pad and its underlying substrate, extended thermal cycling will fatigue the solder and may cause it to crack and eventually fail. A satellite, for example, will thermal cycle between -180.degree. C. and +80.degree. C. for approximately 1600 cycles in a geosynchronous orbit and will thermal cycle between -80.degree. C. and +100.degree. C. for approximately 30,000 cycles in a low earth orbit. Furthermore, direct exposure to an extremely high temperature may liquify the solder causing the joint to fail. The potential for extremely high temperatures exists in military satellites, space probes which utilize solar array aerobraking and some automotive applications. To satisfy the reliability requirements in these types of thermally stressful environments, a non-solder wire termination is required.
The wires, either stranded wire or solid wire, can be terminated to a conductive pad by direct welding. Welding stranded wire to the conductive pad is difficult because the strands separate and may remain loose, thus risking a short circuit. Ensuring each strand is welded is an expensive labor intensive process. Solid wires such as diode or other component leads are difficult to weld because they are typically round and relatively thick, approximately 0.1 cm. Because they are round, the leads present only a small surface contact area with the conductive pad, which weakens the weld joint. Furthermore, the leads cannot be pressed sufficiently to reduce their thickness without sacrificing material integrity. As a result, a high power weld that exerts an extremely high localized pressure, for example 10,000 lbs per square inch (psi), on the underlying substrate is required. The weld may damage the underlying solar panel substrate which is typically formed from a thin brittle material such as graphite or Kevlar.RTM..
A known approach is to crimp a flat lug onto the end of the wire and then weld the flat lug onto the surface of the conductive pad. There are several disadvantages to this process. First, a crimp lug that is strong enough to mechanically hold the wire strands must be relatively thick, e.g. 0.015 to 0.025 cm. The crimp lug is welded to the conductive pad using a pincher weld, in which opposing weld tips are placed on the surface of the lug and underneath the conductive pad, respectively. Because the lug is so thick, a force of approximately 10,000 psi has to be exerted on the weld tips to successfully weld the crimp lug to the conductive pad. Once the conductive pad is mounted onto its substrate, any repairs must be done using a parallel gap weld from the top surface. The extreme force required during welding can damage the thin, brittle graphite or Kevlar.RTM. substrates below the conductive pad. Furthermore, crimping the wire strands is an expensive labor intensive process, in which it is difficult to capture every strand. Stray strands can potentially cause a short. Because this approach involves two connections, pad-to-lug and lug-to-wire, the overall reliability of the wire termination is reduced. In addition, the crimped lug increases the series resistance of the connection.